1. Field of the Invention
This invention relates to a combustion chamber structure of an internal combustion engine, and more particularly to a structure of a pent-roof shaped combustion chamber of an internal combustion engine.
2. Description of the Prior Art
In order to improve the volumetric efficiency to increase the engine output power, it has been known to enlarge the area of the intake port or the like or to increase the number of the valves. For this purpose, a pent-roof shaped combustion chamber is generally employed because of the high degree of freedom it allows in the selection of the area of the valves. In order to improve combustion in the pent-roof shaped combustion chamber, there has been proposed forming the piston head with a bulge that conforms to the shape of the combustion chamber and to form a bowl in the bulged portion to define the combustion chamber, thereby making the combustion chamber compact and at the same time forming a squish area between the piston and the roof surfaces of the pent-roof shaped combustion chamber. See Japanese Unexamined Utility Model Publication No. 59(1984)-96323, for instance.
However, such a combustion chamber structure gives rise to a problem that the squish flow is insufficient at opposite end portions of the edge of the pent-roof at which two roof surfaces merge, and unburned gasses remain at the portions thereby increasing the HC component in the exhaust gas. That is, since the ends of the edge of the pent-roof shaped combustion chamber are defined by vertical walls extending in the sliding direction of the piston, it is difficult to form an effective squish flow between the vertical walls and the piston.
This problem may be overcome by extending the bowl formed in the bulged portion of the piston to the ends of the edge of the combustion chamber. However, this approach is disadvantageous in that the irregular shape of the bowl can adversely affect swirl formation in the combustion chamber. Though swirl in the combustion chamber can be enhanced by selective arrangement of the intake port, this approach can give rise to another problem of pumping loss being increased due to increased intake resistance, thereby adversely affecting fuel economy.